The present invention pertains generally to a sol-gel process for preparing vanadium dioxide. In particular, this process is useful for the preparation of vanadium dioxide thin films.
Traditionally, vanadium dioxide thin films have been prepared either by vacuum deposition or chemical vapor deposition techniques. A gelation-hydrolysis process followed by post-reduction has been used to grow vanadium dioxide thin films from vanadium (V) oxytriisopropoxide as described in "Undoped and Doped VO.sub.2 Films Grown from VO(OC.sub.3 H.sub.7).sub.3 ", Thin Solid Films, by C. B. Greenberg, Vol. 110 (1983) page 73. However, thin films of vanadium dioxide are not easily prepared by these techniques and precise control of deposition parameters is needed to maintain the proper chemical composition. Moreover, post-reduction of high oxide films is difficult to control and tends to leave films inhomogeneous and porous.
A dip coating process for fabricating thin indium-tin oxide films by a sol-gel method is the subject of "Sol-Gel Derived Transparent IR-Reflecting ITO Semiconductor Coatings and Future Applications", by N. J. Arfsten, Journal of Non-Crystalline Solids, Vol. 63 (1984) pages 243-249. However, no procedure for the for the actual preparation of and/or the identification of any starting materials is given.
Dip coatings are also the subject of "Amorphous and Crystalline Dip Coatings Obtained From Organometallic Solutions: Procedures, Chemical Processes and Products", by Helmuth Dislich and Eckart Hussmann, Thin Solid films, Vol. 77 (1981) pages 129-139. While the basic process is broadly described, there is no teaching for the preparation of vanadium dioxide.
U.S. Pat. No. 2,180,353 relates to a process for the preparation of vanadium oxides using a pentavalent vanadium compound as a starting material. According to this process, the pentavalent compound is partially reduced by heating with hydrochloric acid to produce a compound containing vanadium in the tetravalent state. The resultant mixture is then treated with water, heated and ignited to produce a catalytic mixture of oxides (page 2, column 2, lines 19-35). The reduced compound is in the gelatinous state. Note that heating and boiling are required to obtain the gel (page 2, column 1, lines 32-39). Vanadium dioxide is not specifically mentioned.
U.S. Pat. No. 4,684,385 relates a sol gel process wherein a silicon alkoxide/water/alcohol solution is hydrolyzed to form a gel. The gel is then dried to form a glass body (column 1, lines 36-39).
The vanadium dioxide semiconductor to metal phase transition observed at 68.degree. is known to be old in the art.
Other U.S. patents generally related to this technology are U.S. Pat. Nos. 1,941,557; 2,081,272; 3,759,683; 3,847,583; 4,524,051; and 4,539,309.
Thin films of stoichiometric vanadium dioxide are not easily prepared. In chemical vapor deposition and vacuum deposition processes, precise control of the deposition atmosphere is needed to maintain the proper chemical composition. In processes requiring post reduction of higher oxides, the films tend to be inhomogeneous and porous.